During a telephone conversation on January $1$ John tells Mary he will sell her his car for $$500.$ Mary says, "I need some time to think it over" and John said "sure, let me know by January $10."$ Which of the following is $($are$)$ true?

$8$

If John wants to revoke his offer on January $5,$ he may do so if Mary does not accept before that date.

John legally cannot revoke this offer.

An offer has not been made because the terms were orally stated over the telephone.

None of the above.

Both a and c$.$